Chapter 6
Enroute Flight and Navigation

Enroute Flight

When it becomes necessary to divert to an alternate airport, select the most suitable alternate, estimate the magnetic course to the alternate, and turn to the approximate heading to establish this course immediately. After becoming established on the new course, apply the new wind correction and compute the actual distance, estimated time, and fuel required.

Aeronautical charts are drawn using the geographic north pole as the north reference. However, a magnetic compass points to the magnetic north pole, which is not located at the geographic pole. The compass error caused by the angular difference between true north and magnetic north is called magnetic variation. Variation changes with the location on the earth, but it is the same on all headings within the location.

Aeronautical charts show the variation error with isogonic lines that run diagonally and irregularly across the charts. Each line is labeled with the variation error that runs along it, and the pilot must apply the correction for this error to relate the magnetic course to the true course. Westerly variation must be added to the true course or true heading to get the magnetic course or heading. Easterly variation must be subtracted from the true course or true heading to get magnetic course or magnetic heading.

To determine the true heading required to fly a desired true course, crab the airplane into the wind. If the wind is from the left, subtract the wind correction angle. If the wind is from the right, add the correction. See Figure 6-1.

Figure 6-1. Course/heading formula

The most widely used charts for VFR navigation of small, slow airplanes are Sectional Charts. These charts are drawn to a scale of 1:500,000 (1 inch = 6.89 nautical miles), and are revised every six months, except for some Alaskan charts which are revised every 12 months. Sectional Charts are drawn on a grid in which the meridians of longitude are non-parallel straight lines that encircle the earth from pole to pole and cross the equator at right angles. When planning a long distance flight, measure the true course from the mid-meridian to get an average true course.

The Wind Triangle

True course is determined by measuring the course on an aeronautical chart. True airspeed is known by applying the appropriate correction to the indication of the airspeed indicator. The wind direction and velocity are known from reports or forecasts from the Flight Service Stations.

The true heading and the ground speed can be found by drawing a wind triangle of vectors. One side of the triangle is the wind direction and velocity, one side is the true heading and true airspeed, the final side is the track, or true course, and the ground speed. Each side of a wind triangle is the vector sum of the other two sides.

The Flight Computer

Off-Course Correction

After flying for a given distance, you find yourself a known distance off course. You can find the correction angle needed to return the airplane to the desired course in a specific distance by using this simple relationship:

Problem:

After 141 miles are flown from the departure point, the aircraft’s position is located 11 miles off course. If 71 miles remain to be flown, what approximate total correction should be made to converge on the destination?

Solution:

1. Find the change in heading needed to parallel the original course with the formula:

2. Find the change in heading needed to converge on the destination in 71 miles with the formula:

3. Add these two corrections to find the total correction required to converge on the destination:

4.68 + 9.29 = 13.97°

Very High Frequency Omni-Directional Range (VOR)

Radio signals in the very high frequency (VHF) band are nominally in the frequency range of 30 to 300 megahertz (MHz). They operate according to the line-of-sight principle, following the same rules as light, and do not bend to conform to the surface of the earth. VHF reception distance varies in proportion to the altitude of the receiving equipment. The higher the aircraft, the greater the reception distance.

VHF Omni-Directional Range stations (VORs) operate within the 108.0 to 117.95 MHz frequency band and have a power output necessary to provide coverage within their assigned operational service volume. They are subject to line-of-sight restrictions and their range varies with the altitude of the receiving equipment. There are three classes of VOR:

• HVOR Range below 18,000 feet is 40 miles, range between 18,000 feet and FL450 is 130 miles, range above FL450 is 100 miles.

• LVOR Range below 18,000 feet is 40 miles.

• TVOR Range below 12,000 feet is 25 miles.

To orient where the aircraft is in relation to the VOR, first determine which radial is selected (look at the OBS setting). Next, determine whether the aircraft is flying to or away from the station (look at the TO/FROM indicator), to find which hemisphere the aircraft is in. Last, determine how far off course the aircraft is from the selected course (look at the CDI needle deflection) to find which quadrant the aircraft is in. Remember that aircraft heading does not affect orientation to the VOR. If the station is directly abeam of the aircraft, the TO/FROM Indicator will show a neutral flag.

Time and Distance to the Station Using VOR 

The time to the station is found by the formula:

The distance to the station is found by the formula:

Problem:

While maintaining a magnetic heading of 060° and a true airspeed of 130 knots, the 150° radial of a VOR is crossed at 1137 and the 140° radial at 1145. What is the approximate time to the station?

Solution:

Problem:

Given these conditions, find the distance to the station.

Solution:

Therefore, the station is 104 nautical miles away.

VORTAC

Note: Sport Pilot Instructors can disregard this section.

A VORTAC station is a VOR station collocated with a military TACAN station. Civilian airplanes use the VOR for direction to or from the station, and the DME portion of the TACAN for distance information to the station. When a VORTAC station is undergoing routine maintenance, the identification code is removed. The VORTAC provides three individual services:

1. VHF VOR azimuth

2. UHF TACAN azimuth

3. UHF TACAN distance information (DME)

Distance Measuring Equipment (DME)

The mileage readout of DME is the direct distance in nautical miles between the aircraft and the DME ground facility. This is commonly referred to as the slant-range, or slant-line, distance. The slant-range error is greatest when the aircraft is directly above the facility at a high altitude. An aircraft flying at 6,000 feet AGL directly above the facility would indicate a DME distance of 1 nautical mile.

DME furnishes distance information with a high degree of accuracy. Reliable signals may be received at distances of up to 199 nautical miles at line-of-sight altitude, with an accuracy of better than 1/2 mile or 3% of the distance, whichever is greater.

The DME or TACAN coded identification is transmitted 1 time for each 3 or 4 times that the VOR or localizer coded identification is transmitted. When either the VOR or DME is inoperative, it is important to recognize which identifier is retained for the operative facility. A single coded identification with a repetition interval of approximately 30 seconds indicates that the DME is operative.

When in the VOR mode, lateral deflection of the needle in the CDI represents degrees the aircraft is off course. A full needle deflection indicates that the airplane is 10° or more off course.